# std::ranges::view_interface<D>::operator bool

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< cpp‎ | ranges‎ | view interface

constexpr explicit operator bool() requires /* see below */; | (1) | (since C++20)
constexpr explicit operator bool() const requires /* see below */; | (2) | (since C++20)

The default implementation of operator bool member function checks whether the view is non-empty. It makes the derived type contextually convertible to bool.

1) Let `derived` be static_cast<D&>(*this). The expression in the requires-clause is equal to requires { ranges::empty(derived); }, and the function body is equivalent to return !ranges::empty(derived);.
2) Same as (1), except that `derived` is static_cast<const D&>(*this).

### Parameters

(none)

### Return value

false if the value of the derived type is empty (determined by std::ranges::empty), true otherwise.

### Notes

In C++20, no type derived from std::ranges::view_interface in the standard library provides their own operator bool. Almost all of these types use the default implementation.

A notable exception is std::ranges::basic_istream_view. For its iterator type never satisfies `forward_iterator`, the view cannot use the inherited operator bool.

### Example

Run this code

    #include <array>
    #include <iostream>
    #include <ranges>

    int main()
    {
        const std::array ints {0, 1, 2, 3, 4};
        auto odds = ints | std::views::filter([](int i) { return 0 != i % 2; });
        auto negs = ints | std::views::filter([](int i) { return i < 0; });
        std::cout << std::boolalpha
                  << "Has odd numbers: " << (!!odds) << ' ' << '\n'
                  << "Has negative numbers: " << (!!negs) << ' ' << '\n';
    }

Output:

    Has odd numbers: true
    Has negative numbers: false

### See also

ranges::empty (C++20) | checks whether a range is empty (customization point object)
empty | returns whether the derived view is empty. Provided if it satisfies `sized_range` or `forward_range`. (public member function)
empty (C++17) | checks whether the container is empty (function template)

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